The truth about energy-efficient windows

Tony Trobe

In the final instalment of a two-part series, this week Tony Trobe talks again to Dr Peter Lyons, a building physicist based in Canberra. Dr Lyons specialises in energy performance modelling of fenestration systems and buildings, both residential and commercial. One of his continuing research interests involves the impact of glazing on human comfort in buildings.

TT: Last week I talked to Peter Lyons about residential windows, energy efficiency and thermal comfort. This week we look in a bit more detail at today’s windows. Peter, what window materials and technologies allow the high thermal performance we talked about last week?

PL: First, the glass. A sealed, insulating glass unit (IGU) is the basis of a good window. Either double or sometimes triple-glazed, an IGU permits best use of low-emissivity coatings (thin film coatings on the glass you can barely see but which assist the glass to insulate) and also argon gas filling in the gap, instead of just dry air. Low-E coatings are literally "heat mirrors" which reflect heat back into the house and make the window more insulating. Different low-E coatings are available.

Generally in Canberra it is best to use the kind that transmits most solar radiation - for passive solar applications. However there are also coatings that transmit little radiant heat - for that large west-facing window. Second, the frame: look for an insulating frame, such as thermally broken aluminium, timber or uPVC. Composite frames are also available that combine several of these materials. Good glass needs a good frame; in the same way you wouldn't build a Ferrari and then fit it with cross-ply tyres and drum brakes.

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TT: What about thermal comfort? How does it relate to energy efficiency and window materials?

PL: The simple aim is to get the inside window temperature as close as possible to desired room temperature. This means the glass is as warm as possible in winter and as cool as possible in summer. The closer you sit to the window, the more this matters. Fortunately the same technologies that reduce unwanted heat loss or heat gain will also improve the glass temperature, and in the right direction.

TT: Should I use single or double glazing on the north side of my house?

PL: On a sunny day, north-facing windows are valuable since they are your solar collectors which give free heat in winter. But at night they lose a lot of heat compared with an insulated wall. On overcast, cold days the heat loss is nearly as bad. If your budget permits, you are definitely better off with double rather than single glazing, even on the north side, provided its solar heat gain coefficient (SHGC) is high. This is because even though the solar heat gained through double glazing is a little less than through single, the heat losses on a cold day or at night are greatly reduced. On a seasonal basis, in net terms, you will be way ahead with double compared with single glass. In addition, thermal comfort is better with double glazing because it is warmer. Finally, for the same reason, condensation is less likely.

TT: Where is more information available?

PL: Check out www.wers.net, which is a website of the Australian Window Association. Based on independently audited calculations, it lists thousands of energy-rated windows and includes their U-values, SHGCs and star ratings for both winter and summer performance.